This invention relates in general to ultrasound systems, and in particular to an ultrasound system for measuring fluid flow.
Volume flow measurement in medical ultrasound may be important for medical diagnosis. The volume flow may indicate blockage in blood vessels and performance of diseased or transplanted organs. Thus it may be important to determine whether the blood flow out of a kidney has increased or decreased from the previous week.
At present, accurate measurements of blood flow are not attainable on a real-time basis without the use of invasive procedures. Because of the risks involved, these procedures are usually not used to monitor patients on a continuing basis.
Non-invasive ultrasound techniques for measuring volumetric blood flow have been described in "Doppler Ultrasound Physics, Instrumentation, and Clinical Applications," by D. H. Evans et al., John Wiley & Sons, New York, Chapter 11, pages 188-205, 1989. FIG. 1 is a simplified representation of FIG. 11.1 of Evans et al. and illustrates the techniques described by Evans et al. FIG. 1 illustrates an image of an artery showing the placement of the sample gate a and a', the cursor b used to measure the angle between the axis 1 of vessel 2 and the ultrasound beam 4 and the cursors c and c' used to measure the vessel diameter. As shown in FIG. 1, by orienting the scan plane of the ultrasound transducer in the plane that intersects the blood vessel 2 along the length of the vessel, a gray-scale B-mode image of the vessel on the scan plane is obtained as shown in FIG. 1. A cross-sectional dimension of the blood vessel 2 can be obtained by measuring the distance between c' and c. Then assuming that vessel 2 has a circular cross-section, the cross-sectional area of the vessel can be calculated. An ultrasound scan line 4 intersects vessel 2 and a value for the component of the velocity of blood flow 6 in the direction of the scan line 4 can then be obtained from the ultrasound measurement. The real velocity of blood flow can only be determined if the angle .theta. between the scan line 4 and the blood flow direction 6 is known. As taught by Evans et al., assuming that blood flow 6 is along the vessel axis 1, this can be determined from the B-mode image in FIG. 1 by measuring the angle between scan line 4 and a representative axis of vessel 2 to obtain the angle .theta..
As noted by Evans et al., however, the above-described method can result in large errors under unfavorable circumstances. For example, the blood vessel may not have a circular cross-section. Furthermore, the blood flow may not have uniform velocity across its cross-section so that the velocity at the center of the vessel is higher than at the periphery. This means that if a narrow ultrasound beam is used to measure the velocity at the center of the vessel and this measured velocity is taken as the mean velocity, this can lead to an overestimation of the mean velocity of blood flow.
None of the above-described conventional methods for measuring blood flow is entirely satisfactory. It is therefore desirable to provide an improved ultrasound system for measuring blood flow.